How to combine row + columm in a pie box?
The .split() is meaningless for pie menus, it has no effect. When you assign another item to the pie it will be at another position (so top).

To fix it you need to group everything inside a column and create the gap with .separator():

    # ...
    group = pie.column()

    box = group.box()
    row = box.row()
    row.prop(mesh, "use_auto_smooth")
    row.prop(mesh, "auto_smooth_angle", text="Angle")

    group.separator()

    box = group.box()
    box.operator("mesh.faces_shade_smooth", text="Mesh Smooth")
    box.operator("mesh.faces_shade_flat", text="Mesh Flat")
    box.separator()
    box.operator("mesh.normals_make_consistent", text="Normals Outside").inside=False
    # ...